JP7561859B2 - Bistable shape memory alloy inertial actuator - Google Patents

Description

The present invention relates to a bistable current-controlled inertial actuator, its method of operation and its use in a device, in particular an actuator in which the driven element is moved by one or more wires made of a shape memory alloy (hereinafter "SMA").

The shape memory phenomenon is known to consist in the fact that a mechanical piece made of an alloy exhibiting this phenomenon is able to transition, upon a change of temperature, between two shapes preset during manufacture, in a very short time and without intermediate equilibrium positions. The first mode in which the phenomenon can occur is called "one-way", in that the mechanical piece changes shape in one direction upon a change of temperature, i.e. transforms from shape A to shape B, while the transition in the opposite direction, from shape B to shape A, requires the application of a mechanical force.

Alternatively, in the so-called "two-way" mode, both transitions can be induced by temperature changes, and this is the mode intended to be applied in the present invention. This occurs due to a transformation of the crystallite structure of the elements from a type stable at low temperatures, called martensitic, to a type stable at high temperatures, called austenitic, and vice versa (M/A and A/M transitions).

SMA wire needs to be trained to function as a shape memory element. The SMA wire preparation process typically produces, with good repeatability, a martensite/austenite (M/A) phase transition when the wire is heated, and an austenite/martensite (A/M) phase transition when the wire is cooled. The M/A transition causes the wire to shorten by 3%-5%, but when the wire cools, the A/M transition restores this shortening and the wire returns to its original length.

This property of SMA wires, contracting when heated and re-extending when cooled, has long been exploited to obtain very simple, compact, reliable and inexpensive actuators. In particular, actuators of this kind are used in some bistable electrical switches to move a drive element from a first stable position to a second stable position and vice versa. The term "drive element" is intended here to have a very general meaning, since it can take a myriad of forms depending on the specific manufacturing needs, as long as its movement is the element that determines the switching of the switch between two stable operating positions.

Another application of SMA wire is described in US Pat. No. 5,399,633, which discloses a fluid delivery device in which a plunger is moved from a first limit to a second limit by an actuator consisting of an SMA and a biasing spring, without achieving bi-stable operation.
Some examples of bistable SMA wire actuators are described in the applicant's US Pat. Nos. 5,399,433 to 5,633,641, all of which refer to a solution using two SMA wires.

One major drawback common to all these solutions is that actuation of the SMA wire is temperature based and therefore cannot prevent unintended actuation related to changes in the surrounding environment. This problem can become very severe when the SMA-based actuator is in a device where the temperature increases during operation.

Another example of a bistable SMA actuator is described in US Pat. No. 6,399,433, where bistable operation is achieved by using SMA wires in an antagonistic configuration.

An SMA-based solution utilizing a different principle is described in US Pat. No. 5,399,633, which discloses an inertial actuator in which the inertial mass is decoupled from the actuator body and driven over a longer distance by impulse activation. In this invention, certain applications, such as flow diverters described below, involve a high degree of customization to properly design the return mechanism, unavoidable delays when the system needs to be returned to the starting position, and a lack of "symmetry" between the two stable configurations.

International Publication No. 2004/032994 U.S. Pat. No. 4,544,988 U.S. Pat. No. 5,977,858 U.S. Pat. No. 6,943,653 European Patent No. 2735013 U.S. Pat. No. 4,965,545 U.S. Pat. No. 8,656,713 JP 2003-278051 A U.S. Pat. No. 9,068,561 U.S. Pat. No. 6,835,083

“The Mechanical Response of Shape Memory Alloys Under a Rapid Heating Pulse” by Vollach et al published in 2010 on Experimental Mechanics “High-speed and high-efficiency shape memory alloy actuation” by Motzki et al. published in 2018 on Smart Materials and Structures “A Study of the Properties of a High Temperature Binary Nitinol Alloy Above and Below its Martensite to Austenite Transformation Temperature” by Dennis W. Norwich presented at the SMST 2010 conference

The object of the present invention is to overcome the drawbacks of the prior art with a solution that allows rapid switching and preventing accidental actuation, the first aspect of which is a bistable SMA inertial actuator comprising a body connected to one or more SMA wires actuable by short current pulses. The one or more SMA wires are in operative contact with an inertial element having a diameter of 0.025 mm to 0.5 mm and a mass M that moves between a first stable position and a second stable position and vice versa under simultaneous actuation of the one or more SMA wires. The inertial actuator is subjected to an elastic locking force with an elastic constant K expressed in g/mm, which in the most preferred embodiment is provided by a spring attached to the body.

The actuator has a characteristic parameter P defined as the inverse of said elastic constant, i.e. 1/K expressed in mm/g, multiplied by the ratio between the mass M of the inertial element expressed in grams and the total cross-sectional area A expressed in ^mm2 of the SMA wire in operative contact with the inertial element of mass M, i.e. P=(1/K)*(M/A). The inventors have surprisingly discovered that the system is essentially invariant with respect to the wire diameter, the elastic constant K and the inertial element mass M, in that a bistable behavior can be established as long as the characteristic parameter P defined above is comprised within a certain range, i.e. between 15 mm ⁻¹ and 750 mm ⁻¹ , preferably between 25 ^{mm −1} and 500 mm −1 .

The source of the current pulse is not the subject of this invention and is well known in the art, it may be part of the bistable inertial actuator (attached to the body) or simply external, connected to one or more SMA wires by suitable cabling. For example, one of the simplest ways to provide a short actuation pulse to a shape memory element is via a capacitor discharge, as described in the aforementioned US Pat. No. 5,399,363. Such a capacitor can be simply integrated and attached to the actuator body or it can be attached externally to the actuator body.

In the present invention, when multiple SMA wires are in operative contact with an inertial element of mass M, the SMA wires have the same or very similar diameters (±10%); otherwise, the system becomes complicated with respect to current control and regulation of the SMA wires. Furthermore, much of the work is performed by the largest diameter wire, and the additional work has limited operational contribution and introduces additional complexity that is cumbersome to address.

It is important to underline that since all SMA wires simultaneously contribute to the mass displacement of the body, the current configuration maximizes the pulling of the SMA wires and achieves various benefits (higher mass displacement capacity, smaller size, simplified control electronics). All of this cannot be achieved with the structure shown in the aforementioned US Pat. No. 6,333,363, due to the antagonistic configuration of the SMA wires.

It is also important to clarify that actual SMA wire may deviate from a circular cross-section, and therefore the term diameter is intended as the diameter of the smallest enclosing circle.

1 is a schematic view from above of a first embodiment of an actuator according to the invention, with an inertial element in each of its two stable positions; FIG. 1 is a schematic view from above of a first embodiment of an actuator according to the invention, with an inertial element in each of its two stable positions; FIG. FIG. 3 is a schematic view from above of a variant of the actuator of FIGS. 1-2; FIG. 3 is a schematic perspective view of a modified example of the actuator of FIGS. 1 and 2. 1 is a schematic cross-sectional view of an airbag incorporating a bistable SMA inertial actuator according to the present invention; FIG. 2 is a schematic see-through view of a damper incorporating a bistable SMA inertial actuator according to the present invention. FIG. 13 is a schematic cross-sectional view of a flow diverter valve incorporating a bistable SMA inertial actuator according to a second embodiment. FIG. 13 is a schematic cross-sectional view of a flow diverter valve incorporating a bistable SMA inertial actuator according to a second embodiment. FIG. 1 is a schematic perspective view of a padlock incorporating a bistable SMA inertial actuator according to a first embodiment; FIG. 1 is a schematic perspective view of a padlock incorporating a bistable SMA inertial actuator according to a first embodiment;

The invention will be further described with reference to the following figures:

In the figures, the size and dimensional ratios of various elements shown have been altered in some cases for ease of understanding, in particular but not limited to the diameter of the SMA wire relative to other elements of the bistable actuator. Also, some auxiliary elements not necessary for understanding the invention, such as current sources, wire crimping/fixing elements, etc., are not shown as they are conventional means known in the art.

1 and 2 show schematic views from above of a bistable SMA inertial actuator 10 according to a first embodiment of the present invention in two stable positions. On an actuator body 11 having a main/major portion with a circular segment shape, an SMA wire 12 is fixed at points 111, 111' close to the end part of the body 11. The circular segment part of the body 11 has three hinges 15, 16, 16', of which the first hinge 15 is located in the lower central part of the actuator body 11 and allows and facilitates temporary expansion of the diameter of the circular segment part of the body. The other two hinges 16, 16' are symmetrically arranged corresponding to the end parts of the circular segment part of the body. Two arms 17, 17' of equal length pivotally connect the hinges 16, 16' to an inertial element 13 of mass M. A spring 14 having an elastic constant K acting as a locking elastic element connects opposite sides of the circular segment portion (secant) of the body. In the first embodiment of Figures 1 and 2, the spring 14 has a length equal to the diameter, but the length of the spring 14 may be shorter if placed above or below the circular portion of the actuator body.

When the SMA wire 12 is rapidly actuated to shorten it, the circular element portion of the body expands, transmitting an acceleration to the mass M which moves from the internal position shown in FIG. 1 to the external position shown in FIG. 2. The inertial element 13 of the mass M is then held stable and firm in its external position by the action of the spring 14 which pulls the two hinges 16, 16' towards each other. When the wire 12 is actuated again rapidly, the same deformation mechanism of the actuator body 11 accelerates the inertial element 13 back to the internal position shown in FIG. 1.

In this type of actuator configuration, the major portion of the circular segment geometry has a length that is preferably about 2/3 to 5/6 of a complete circle.

The diameter of the circle is preferably comprised between 1-2 mm and 20 cm or more, with variations in the diameter of the circle being made accordingly taking into account different applications that can benefit from the use of the bistable inertial actuator according to the invention, such as small applications such as mobile camera autofocus actuators, and larger applications such as friction adjustment of moving parts of hydraulic equipment such as pistons.

It should be emphasized that the invention is not strictly limited to the elements and configurations as shown in Figures 1 and 2; for example, as a first variant (not shown), the central spring 14 can be replaced with a flat elastic element, such as a flexure, that follows the contour of the actuator body 11.

Another similar variant is shown in Figures 3A and 3B, which show a schematic view from above and a schematic perspective view of a bistable SMA inertial actuator 30, respectively, in which the elastic biasing/locking force is provided to the inertial mass 33 by the actuator body 31 itself, not by its resistance to deformation, i.e., by a flexure due to the expansion of the diameter. Furthermore, this solution does not require a hinge (i.e., element 15 in Figures 1 and 2) located at the bottom of the body 31, but simply hinges 36, 36' for the movement of the inertial element 33 through arms 37, 37' of equal length.

This variation also includes three SMA wires 32, 32', 32'' connected in parallel.

The above embodiments show that there are many variations and possible combinations of elements that fall within the inventive concept of the present invention. For example, another straightforward variation is to use SMA wires in series, i.e. in Figs. 1 and 2, two SMA wires each connected between hinge 15 and points 111, 111', or to use multiple wires in a given pattern embedded in a suitable retaining element, such as a cloth, as described in, for example, US Pat. No. 5,399,363.

Moreover, preferably, the length of the path connecting the hinges 16, 16' along the two arms 17, 17' via the inertial element 13, and the length of the path connecting the hinges 36, 36' along the two arms 37, 37' via the inertial element 33 are between 1.1 and 3 times the length of the straight line connecting the hinges.

As already mentioned, actuators made according to the present invention are able to prevent unintended actuation related to external factors such as environmental temperature fluctuations. This aspect is of paramount importance in a wide range of applications where accidental actuation may not only be detrimental to the operation of the device but may also pose a safety hazard, airbags being one such application where this issue needs to be adequately addressed.

In this regard, FIG. 4 shows a pressurized part 40 of an airbag incorporating a bistable SMA inertial actuator 10 according to the embodiment shown in FIGS. 1 and 2. Such an actuator 10 is mounted on a frame 41 provided with a breakable seal 44 and having a channel 42 communicating between a sealed and pressurized gas container 43 and the airbag to be inflated (not shown). The bistable SMA inertial actuator 10 is inserted between a loaded spring 45 and a punch 46, which, when released, acts on the punch 46 to break the seal 44 upon (high speed) actuation of the SMA wire 12. As already mentioned, this application greatly benefits from the use of a bistable SMA inertial actuator according to the invention, since it prevents unintended actuation.

The application of the bistable SMA inertial actuator 10 according to the invention in different types of devices is shown in FIG. 5. In this case, the damper 50 has a fixed frame 51 and is attached to a shaft 54 with a fixed end 52 and a movable end 53 that slides within the frame 51 through suitable openings. The friction and damping behavior encountered by the sliding shaft 54 in vertical movement/oscillation is accommodated by the bistable SMA inertial actuator 10, which applies a higher compression to the moving shaft in one of two stable positions, increasing the applied frictional force and thus improving stiffness.

6 and 7 show a flow diverting valve 600 having an inlet 601 and two outlets 602 and 602' selectively communicated with the inlet 601 by a bistable SMA inertial actuator 60 including an SMA wire 62 operatively connected to an inertial mass 63, acting alternately as a shutter for each of the two outlets. A first arm 66 connects the inertial mass 63 to a pivot 65 and slides within the actuator body 61, around which the SMA wire 62 is wound. A second arm 67 connects the inertial mass 63 to a fixed pivot 68, said arms 66, 67 being attached to the inertial mass 63 via a pivoting joining element 69. The second arm 67 also comprises a slot 67a in which an element 69 can slide to allow rotation of the arm 67 around a fixed pivot 68, the elastic locking force being provided by a biasing spring 64 acting on the sliding pivot 65 in a direction opposite to the action of the SMA wire 62. When the SMA wire 62 is actuated at high speed, the inertial mass 63 slides vertically, thus opening one of the outlets 602 and 602' and acting as a shutter for the other outlet. This is another application that greatly benefits from the possibility of high speed actuation to initiate the inertial mass movement, since actuation is independent of the temperature of the regulated flow.

The same principles detailed in Figures 6 and 7 for fluid communication within a valve can also be applied in other areas, such as electronic circuits for low voltage/high voltage switches, or mechanical locks that allow physical access to a key lock opening to insert a key, thereby providing an increased level of security.

Another application in which the bistable actuator of the present invention can be advantageously applied is smart locks such as padlocks. SMA inertial bistable padlocks are devices that overcome the limitations of standard SMA padlocks by taking advantage of the fast actuation to move one or more bistable mechanisms to lock/unlock the padlock's steel shackle.

In fact, SMA inertial mechanisms are not subject to thermal self-actuation because the heating rate imparted by a flame or another external heat source in direct contact with the padlock case is not fast enough to switch the state of the bistable mechanism. Furthermore, it is possible to duplicate or triplicate the mechanism with different working directions (e.g., x, y, or z) and/or application directions (north-south, north-south) to avoid mechanical shock opening. Since SMA inertial padlocks are electrically actuated, electrical inputs can be controlled directly, e.g., by fingerprint, face recognition, key, or indirectly, e.g., by Wi-Fi, Bluetooth, RFID, GSM.

The use of the bistable SMA actuator according to the first embodiment in a padlock is shown in the perspective views of Figures 7 and 8. The padlock 700 includes a solid body 701 that holds a sliding shackle 702, and a bistable inertial SMA actuator 10 with a locking pin 10' that engages a recess 702' in the shackle, as shown in Figure 7. Figure 8 shows the bistable inertial SMA actuator 10 in a second stable position with the locking pin 10' disengaged from the recess 702' so that the shackle 702 can slide out freely from the padlock body 701.

The padlock 700 of Figures 8 and 9 has a sliding shackle 702 that can be fully removed from the padlock body 701, but a direct variant (not shown) envisages a sliding shackle with legs of different lengths, where partial sliding of the shackle allows the shorter legs to move freely, opening the padlock while retaining the shackle on the longer legs with a suitable stop.

It should be emphasized that a person skilled in the art would understand how to achieve rapid actuation of SMA wires, see for example Non-Patent Document 1 or Non-Patent Document 2.

By actuation time is intended the time required to bring the SMA wire to a temperature in the austenitic phase, the so-called Af temperature. To achieve such an effect, even in the case of thin wires, such as wires with a diameter of less than 100 μm, some electronic circuitry may be associated with the SMA wire current supply, such as a capacitor, and even a battery can achieve such short actuation times.

The actual parameter that drives and determines the actuation time is the actuation pulse duration, i.e. the time during which current is supplied to the SMA wire. Such an actuation pulse duration is consequently much shorter than the actuation time as defined above.

In its second aspect, the invention relates to a method for operating a bistable SMA inertial actuator, more specifically a method in which the actuation pulse is short, i.e. the duration of the current applied to the SMA wire is between 0.1 ms and 50 ms, preferably between 1 ms and 25 ms.

The present invention is not limited to a particular SMA material, but Ni-Ti based alloys such as Nitinol, which may alternatively exhibit superelastic wire behavior or SMA behavior depending on the processing, are preferred. The properties of Nitinol and the methods that make it possible to achieve them are widely known to those skilled in the art, see for example Non-Patent Document 3.

Nitinol can be used as is, or its transition temperature characteristics can be adjusted by adding elements such as Hf, Nb, Pt, Cu, etc. The appropriate selection of material alloys and their properties are generally known to those skilled in the art, see, for example:
http://memry.com/nitinol-iq/nitinol-fundamentals/transformation-temperatures

SMA wires can also be used "by themselves" or with a coating/sheath to improve their thermal management, i.e. their cooling after actuation. The coating sheath can be uniform as described in US Pat. No. 5,399,366, which teaches how to manage residual heat by using an electrically insulating coating that is a thermal conductor, but US Pat. No. 5,399,366 discloses that the SMA wire is provided with an enclosing sheath that can improve cooling after every actuation cycle. Also, coatings with a suitable dispersion of phase change material can be used to advantage, as described in the applicant's WO 2019/003198.

A series of experiments was carried out on a prototype bistable actuator of the present invention, made with a single SMA wire 12, a plastic body 11 with a diameter of 155.03 mm, and arms 17, 17' with lengths of 45 mm, as shown in Figure 1.

Different diameters of SMA wire were tested with the same standard Nitinol material at As=95°C, with different masses of inertia, as well as biasing springs with various elastic constants. The results are shown in the table below, with examples made in accordance with the invention denoted with the prefix S and comparative examples outside the scope of the invention denoted with the prefix C.

As can be observed from the various examples above, only combinations with appropriate values of the characteristic parameter P could achieve satisfactory bistable operation, while values above or below certain ranges resulted in unsatisfactory operation.

All results reported in the table above were performed with a pulse actuation period of 13 ms.

The evaluation of the influence of the SMA wire at different pulse actuation periods was carried out on a subset of samples according to the invention, namely samples S1, S4 and S7. More specifically, the behavior of the system was evaluated at very fast actuations (pulse actuation periods <0.1 ms) with the inertial mass bouncing, whereas at very slow actuations (pulse actuation periods >50 ms) the inertial mass was not able to reach the second stable position.

Claims (13)

A bistable SMA inertial actuator (10; 30; 60) comprising an actuator body (11; 31; 61) connected to one or more SMA wires (12; 32, 32', 32"; 62) actuatable by current pulses generated by an electronic circuit,
said one or more SMA wires (12; 32, 32', 32"; 62) being in operative contact with an inertial element (13; 33; 63) which moves between a first stable position and a second stable position and vice versa;
The actuator is subjected to a locking force with an elastic constant K expressed in g/mm (0.0098 N/mm),
said inertial element (13; 33; 63) being movable between said first and second stable positions and vice versa under simultaneous actuation of said one or more SMA wires (12; 32, 32', 32"; 62) having a diameter between 0.025 mm and 0.5 mm,
said actuator having a characteristic parameter P defined as the inverse of said elastic constant multiplied by the ratio between the mass M of said inertial element (13; 33; 63) expressed in grams and the total cross-sectional area A expressed in ^mm2 of said SMA wire(s) (12; 32, 32', 32"; 62) in operative contact with said inertial element (13; 33; 63), where P=(1/K)*(M/A),
The characteristic parameter P is between 23.31 mm ⁻¹ and 653 mm ⁻¹ ;
The bistable SMA inertial actuator, wherein the electronic circuitry is configured to generate current pulses with actuation durations between 0.1 milliseconds and 50 milliseconds. The bistable SMA inertial actuator of claim 1 , wherein the characteristic parameter P is between 25 mm ⁻¹ and 500 mm ⁻¹ . A bistable SMA inertial actuator as claimed in claim 1 or 2, wherein the electronic circuit is configured to generate current pulses with an actuation period between 1 ms and 25 ms. A bistable SMA inertial actuator according to any one of claims 1 to 3, wherein at least one element of the electronic circuit that generates the current pulses is attached to the actuator body (11; 31; 61). The bistable SMA inertial actuator of claim 4, wherein the at least one element of the electronic circuit is a capacitor. A bistable SMA inertial actuator according to any one of claims 1 to 5, comprising a plurality of SMA wires (12; 32, 32', 32"; 62) connected in parallel or series. The bistable SMA inertial actuator of any one of claims 1 to 6, wherein the locking force having an elastic constant K is provided by an elastic element (14; 64) connected to the actuator body (11; 61) or by the actuator body (31) itself. A bistable SMA inertial actuator according to any one of claims 1 to 7, wherein the actuator body (11; 31) has a main portion with a geometry of a circular segment that is 2/3 to 5/6 of a complete circle. The bistable SMA inertial actuator of claim 8, wherein a plurality of hinges (15, 16, 16'; 36, 36') are arranged on the actuator body (11; 31) having a circular segment geometry. Use of a bistable SMA inertial actuator according to any one of claims 1 to 9 to control the actuation of a device. The use of the bistable SMA inertial actuator of claim 10, wherein the device is an airbag (40), a damper (50), a flow diverter valve (600), an electrical circuit switch, a padlock or a lock. A method for operating a bistable SMA inertial actuator according to any one of claims 1 to 9, wherein the duration of the current pulse for actuating the one or more SMA wires (12; 32, 32', 32"; 62) is between 0.1 ms and 50 ms. The method of operating a bistable SMA inertial actuator according to claim 12, wherein the duration of the current pulse for actuating the one or more SMA wires (12; 32, 32', 32"; 62) is between 1 ms and 25 ms.

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